The present invention relates to the field of communication systems, and more particularly to a system and method operable to facilitate wide band optical amplification while maintaining acceptable noise figures.
Because of the increase in data intensive applications, the demand for bandwidth in communications has been growing tremendously. In response, the installed capacity of telecommunication systems has been increasing by an order of magnitude every three to four years since the mid 1970s. Much of this capacity increase has been supplied by optical fibers that provide a four-order-of-magnitude bandwidth enhancement over twisted-pair copper wires.
To exploit the bandwidth of optical fibers, two key technologies have been developed and used in the telecommunication industry: optical amplifiers and wavelength-division multiplexing (WDM). Optical amplifiers boost the signal strength and compensate for inherent fiber loss and other splitting and insertion losses. WDM enables different wavelengths of light to carry different signals in parallel over the same optical fiber. Although WDM is critical in that it allows utilization of a major fraction of the fiber bandwidth, it would not be cost-effective without optical amplifiers. In particular, broadband optical amplifier systems that permit simultaneous amplification of many WDM channels are a key enabler for utilizing the full fiber bandwidth.
Traditionally, amplification of signals having a broad range of wavelengths has required separating the signals into subsets of wavelengths, and amplifying each subset with a separate amplifier. This approach can be complex and expensive. Using separate amplifiers for each subset requires additional hardware, additional laser pumps for each amplifier, and additional power to launch the additional pumps.
Although a more efficient approach would be to amplify the entire signal using a single amplifier for at least some amplifiers in the system, unfortunately, no acceptable single amplifier approach has been developed. For example, erbium doped-amplifiers are an inherently bad choice for wide band amplification if the ultimate goal is to provide an amplifier that can operate over the entire telecommunications spectrum. For example, for wavelengths shorter than about 1525 nanometers, erbium-atoms in typical glasses will absorb more than they amplify. Even with use of various dopings, such as, aluminum or phosphorus, the absorption peak for the various glasses is still around 1530 nanometers. This leaves a large gap in the short communications band (S-Band) unreachable by erbium doped fiber amplifiers.
Raman amplifiers provide a better solution in terms of broadband amplification potential, but conventional Raman amplifiers have suffered from other shortcomings. For example, Raman amplifiers have traditionally suffered from high noise figures when used in wide band applications. In addition, Raman amplifiers suffer from gain tilt introduced when longer wavelength signals rob energy from shorter wavelength signals. This effect becomes increasingly pronounced as amplifier launch power and system bandwidth increases. Wide band Raman amplifiers operating at high launch powers on a wide range of wavelengths can be particularly vulnerable to this effect.
Masuda, et al. (see e.g., U.S. Pat. No. 6,172,803 B1 and related research papers) have attempted to improve the bandwidth of erbium doped amplifiers by cascading with the erbium doped amplifier a Raman amplifier with an approximately complementary gain profile. Masuda, et al, however, consistently require the presence of an erbium doped amplifier (which relies on different physics for amplification and does not suffer from the same noise problems as Raman amplifiers do) to provide virtually all amplification to signal wavelengths close in spectrum to the pump wavelengths. Indeed, Masuda, et al. concede that the noise figures they report ignore the effect of the Raman portion of their amplifier.
The present invention recognizes a need for a method and apparatus operable to facilitate wide band Raman amplification while maintaining an approximately flat gain profile and an acceptable noise figure.
In accordance with the present invention, a system and method for providing wide band Raman amplification are provided that substantially reduce or eliminate at least some of the shortcomings associated with prior approaches. In one aspect of the invention, a multi-stage Raman amplifier comprises a first Raman amplifier stage having a first sloped gain profile operable to amplify a plurality of signal wavelengths, and a second Raman amplifier stage having a second sloped gain profile operable to amplify at least most of the plurality of signal wavelengths after those wavelengths have been amplified by the first stage. The second sloped gain profile has an approximately complementary slope to the slope of the first sloped gain profile. The combined effect of the first and second Raman stages contributes to an approximately flat overall gain profile over the plurality of signal wavelengths.
In another aspect of the invention, a method of amplifying an optical signal having multiple wavelengths comprises amplifying a plurality of signal wavelengths at a first Raman amplifier stage having a first sloped gain profile, and amplifying at least most of the plurality of signal wavelengths at a second Raman amplifier stage after those signal wavelengths have been amplified by the first stage. The second stage has a second sloped gain profile comprising an approximately complimentary gain profile to the first gain profile. The combined effect of the first and second Raman stages contributes to an approximately flat overall gain profile over the plurality of signal wavelengths.
In still another aspect of the invention, a multi-stage Raman amplifier comprises a plurality of cascaded Raman amplifier stages each having a gain profile, wherein the gain profile of at least some of the Raman stages is sloped. At least two of the sloped gain profiles comprise approximately complimentary gain profiles, wherein the combined effect of the gain profiles of the amplification stages results in an approximately flat overall gain profile over a plurality of signal wavelengths amplified by the amplifier.
In yet another aspect of the invention, a method of amplifying multiple-wavelength optical signals comprises applying a first sloped gain profile to a plurality of signal wavelengths at a first stage of a Raman amplifier, and applying a second sloped gain profile to at least most of the plurality of signal wavelengths at a second stage of the Raman amplifier. The second gain profile comprises an approximately complementary gain profile of the first sloped gain profile. The combined effect of the first and second sloped gain profiles contributes to an approximately flat overall gain profile over the plurality of signal wavelengths.
In another aspect of the invention, a multi-stage Raman amplifier comprises a plurality of cascaded Raman amplifier stages each operable to amplify a plurality of signal wavelengths and each having a gain profile determined at least in part by one or more pump wavelengths applied to the amplifier stage. The plurality of amplifier stages comprise a first Raman stage operable to apply a higher gain level to a signal wavelength closest to a longest pump wavelength than a gain applied to a signal wavelength furthest from the longest pump wavelength.
In still another aspect of the invention, a method of amplifying an optical signal having multiple wavelengths comprises receiving a plurality of signal wavelengths at a plurality of cascaded Raman amplifier stages having at least a first stage and a last stage, where each stage is operable to amplify a plurality of signal wavelengths and each stage has a gain profile determined at least in part by one or more pump wavelengths applied to the amplifier stage. The method further includes applying a highest level of gain supplied by the longest pump wavelength in the last Raman stage of the amplifier.
In yet another aspect of the invention, a multi-stage Raman amplifier comprises a plurality of cascaded Raman amplifier stages, at least some of the Raman stages having sloped gain profiles operable to contribute to a combined gain profile of the amplifier. The combined gain profile of the amplifier is approximately flat across a bandwidth of at least eighty nanometers and comprises a small signal noise figure no greater than eight decibels.
In another aspect of the invention, a method of amplifying an optical signal having multiple wavelengths comprises amplifying a plurality of signal wavelengths at a first Raman amplifier stage having a first sloped gain profile, and amplifying at least most of the plurality of signal wavelengths at a second Raman amplifier stage having a second sloped gain profile that is different than the first sloped gain profile. The combined gain profile of the amplifier is approximately flat across a bandwidth of at least eighty nanometers and comprises a small signal noise figure no greater than eight decibels.
In another aspect of the invention, an optical pre-amplifier operable to be coupled to an optical communication link carrying optical signals having a plurality of wavelengths comprises a first Raman stage having a gain profile where a majority of shorter signal wavelengths are amplified more than a majority of longer signal wavelengths. The preamplifier further comprises a second Raman stage operable to receive at least most of the signal wavelengths after they have been amplified by the first stage, the second stage having a gain profile where a majority of longer signal wavelengths are amplified more than a majority of shorter signal wavelengths. In this embodiment, the gain profiles of the first and second Raman stages are operable to combine to contribute to an approximately flat combined gain profile over the plurality of signal wavelengths.
In still another aspect of the invention, an optical booster amplifier operable to be coupled to an optical communication link carrying optical signals having a plurality of wavelengths comprises a first Raman stage having a gain profile where a majority of longer signal wavelengths are amplified more than a majority of shorter signal wavelengths. The booster amplifier also comprises a second Raman stage operable to receive at least most of the signal wavelengths after they have been amplified by the first stage, the second stage having a gain profile where a majority of shorter signal wavelengths are amplified more than a majority of longer signal wavelengths. The gain profiles of the first and second Raman stages are operable to combine to contribute to an approximately flat combined gain profile over the plurality of wavelengths.
In yet another aspect of the invention, a Raman amplifier assembly comprises a preamplifier coupled to an optical communication link. The preamplifier includes a first Raman stage having a gain profile wherein a majority of shorter wavelengths are amplified more than a majority of longer wavelengths, and a second Raman stage having a gain profile approximately complementary to the first gain stage. The amplifier assembly also includes a booster amplifier coupled to the optical communication link. The booster amplifier comprises a first Raman stage having a gain profile wherein a majority of longer wavelengths are amplified more than a majority of shorter wavelengths, and a second Raman stage having a gain profile approximately complementary to the first gain stage.
In another aspect of the invention, an optical communication system operable to facilitate communication of multiple signal wavelengths comprises a transmitter bank operable to generate a plurality of signal wavelengths, and a multiplexer operable to combine the plurality of signal wavelengths into a single multiple wavelength signal for transmission over a transmission medium. The system further comprises an amplifier coupled to the transmission medium and operable to amplify the multiple wavelength signal prior to, during, or after the multiple wavelength signal""s transmission over the transmission medium, the amplifier comprising a multi-stage Raman amplifier. The amplifier includes a first Raman amplifier stage having a first sloped gain profile operable to amplify a plurality of signal wavelengths and a second Raman amplifier stage having a second sloped gain profile operable to amplify at least most of the plurality of signal wavelengths after those wavelengths have been amplified by the first stage. The second sloped gain profile has an approximately complementary slope to the slope of the first sloped gain profile, and the combined effect of the first and second Raman stages contributes to an approximately flat overall gain profile over the plurality of signal wavelengths. In one embodiment, the system further includes a demultiplexer operable to receive the multiple wavelength signal and to separate the signal wavelengths from the multiple wavelength signal, and a receiver bank operable to receive the plurality of signal wavelengths.
Depending on the specific features implemented, particular embodiments of the present invention may exhibit some, none, or all of the following technical advantages. For example, one aspect of the invention facilitates optical amplification of a wide bandwidth of wavelengths while maintaining an approximately flat gain profile and an acceptable noise figure.
In a particular embodiment, one aspect of the invention reduces the noise figure associated with the amplifier by amplifying in a first Raman stage a majority of shorter wavelengths more than a majority of longer wavelengths. In this way, shorter wavelengths (which are often closest to the pump wavelength) are amplified to overcome any effects that might be caused by phonon-stimulated noise. As a further enhancement, the amplifier could be designed so that the longest pump wavelength is at least ten nanometers below the shortest signal being amplified.
In addition to yielding an acceptable noise figure, this approach can produce an approximately flat gain tilt, for example, by cascading a second Raman amplifier stage having a gain profile that amplifies a majority of longer wavelengths more than a majority of shorter wavelengths. In a particular embodiment, the second gain profile can be approximately complementary to the first gain profile. In some applications, the second gain profile can have an approximately equal (although opposite) slope from the first gain profile.
Another aspect of the invention results in increased efficiency in a multi-stage Raman amplifier. This aspect of the invention involves applying, in at least one Raman stage, a first gain profile that amplifies a majority of longer wavelengths more than a majority of shorter wavelengths; and applying, in a later cascaded Raman stage, a second gain profile that amplifies a majority of shorter wavelengths more than a majority of longer wavelengths. This embodiment facilitates allowing longer pump wavelengths in the first stage to accept energy from shorter pump wavelengths in the later Raman stage. This effect, in turn, facilitates using smaller pump wavelengths and/or fewer pump wavelengths in the first stage than would otherwise be required, thereby increasing the efficiency of the device. In a particular embodiment, the gain profiles of the first and later Raman stages can be approximately complimentary, contributing to an approximately flat overall gain profile for the amplifier. The noise figure can be reduced, for example, by performing a majority of the amplification of wavelengths closest to the pump wavelengths in one of the final amplifier stages, or in the last amplifier stage.
Other aspects of the invention facilitate cascading multiple amplifier stages to realize advantages of low noise and high efficiency in a multiple stage Raman amplifier. Moreover, cascaded stages can provide mid-stage access to the amplifier to facilitate, for example, optical add/drop multiplexing of WDM signals while maintaining an acceptable noise figure and an approximately flat gain profile, both at the mid-stage access point and across the entire amplifier.
Other technical advantages are readily apparent to one of skill in the art from the attached figures, description, and claims.